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Biomedical Imaging I Class 3 – X-Ray CT Instrumentation 9/28/04 BMI I FS05 – Class 3 “CT Instrumentation” Slide 1 X-ray computed tomography Limits of radiography / fluoroscopy 3D structures are collapsed into 2D image (obscuring of details, loss of one dimension) Low soft-tissue contrast Not quantitative Features of x-ray CT X-ray imaging modality (same principles of generation, interaction, detection) Generation of a sliced view of body interior Computed reconstruction of images Good soft-tissue contrast BMI I FS05 – Class 3 “CT Instrumentation” Slide 2 Examples of CT images BMI I FS05 – Class 3 “CT Instrumentation” Slide 3 Principle of x-ray CT In one plane, obtain set of line integrals for multiple view angles Reconstruct cross-sectional views Linear scan Source Angular scan Object Detector BMI I FS05 – Class 3 “CT Instrumentation” Slide 4 Scanner Design BMI I FS05 – Class 3 “CT Instrumentation” Slide 5 Realization of x-ray CT Mathematical basis for computed tomography by Radon (1917) Idea popularized by Allan Cormack at Tufts Univ. (1963) First practical x-ray CT scanner introduced by Godfrey Hounsfield of EMI Ltd., England (1972) BMI I FS05 – Class 3 “CT Instrumentation” Slide 6 First generation EMI Mark I (Hounsfield), “pencil beam” or parallel-beam scanner (highly collimated source) excellent scatter rejection, now outdated 180 - 240 rotation angle in steps of ~1 Used for the head 5-min scan time, 20-min reconstruction Original resolution: 80 80 pixels (ea. 3 3 mm2), 13-mm slice BMI I FS05 – Class 3 “CT Instrumentation” Slide 7 Second generation Hybrid system: Fan beam, linear detector array (~30 detectors) Translation and rotation Reduced number of view angles scan time ~30 s Slightly more complicated reconstruction algorithms because of fan-beam projection BMI I FS05 – Class 3 “CT Instrumentation” Slide 8 Third generation Wide fan beam covers entire object 500-700 detectors (ionization chamber or scintillation detector) No translation required scan time ~seconds (reduced dose, fewer motion artifacts) Reconstruction time ~seconds Pulsed source (reduces heat load & radiation dose) BMI I FS05 – Class 3 “CT Instrumentation” Slide 9 Fourth generation Stationary detector ring (600 – 4800 scintillation detectors) Rotating x-ray tube (inside or outside detector ring) Scan time, reconstruction time ~seconds Source either inside detector ring or outside (rocking, nutating detectors) BMI I FS05 – Class 3 “CT Instrumentation” Slide 10 Comparison of 3rd and 4th generation Both designs currently employed, neither can be considered superior 3rd Generation (GE, Siemens): Fewer detectors (better match, cheaper) Good scatter rejection with focused septa Cumulative detector drift 4th Generation (Picker, Toshiba): Less moving parts Detectors calibrated twice per rotation BMI I FS05 – Class 3 “CT Instrumentation” Slide 11 X-ray CT sources BMI I FS05 – Class 3 “CT Instrumentation” Slide 12 X-ray tubes Bremsstrahlung x-ray tubes Fixed anode: oil-cooled Rotating Anode Two focal spot sizes (~0.5 mm 1.5 mm and ~1.0 mm 2.5 mm) Collimator assembly used to control beam (slice) width (~1.0 - 10 mm) Power: ~120 kV @ 200-500 mA spectrum ~30 – 120 keV High frequency generators (5-50 kHz) Rotating geometry requires slip rings High voltage slip rings (~120 kV) if generator stationary Lower voltage slip rings (480 V) if generator on rotary gantry BMI I FS05 – Class 3 “CT Instrumentation” Slide 13 Rotary Gantry X-ray tube Picker International, Inc. BMI I FS05 – Class 3 “CT Instrumentation” Slide 14 Slip rings Picker International, Inc. BMI I FS05 – Class 3 “CT Instrumentation” Slide 15 X-ray Detectors BMI I FS05 – Class 3 “CT Instrumentation” Slide 16 Detector Performance Desired: High overall efficiency to minimize patient radiation dose (typ. 0.45…0.85) = product of Geometric efficiency: fraction of detector area sensitive to radiation Quantum efficiency: fraction of radiation energy deposited Conversion efficiency: fraction of absorbed radiation contributing to electrical signal Large dynamic range (ratio of largest to smallest detectable signal) Stable in time (low drift) Insensitive to temperature variations BMI I FS05 – Class 3 “CT Instrumentation” Slide 17 Gas ionization chambers I Measurement of conductivity induced in a gas volume by the ionizing effect of x-rays. X-rays ionize gas molecules Ions are drawn to electrodes by electric field Number of ion pairs N produced x-ray intensity Collimator Anode + - - + + + + - Ampmeter Cathode BMI I FS05 – Class 3 “CT Instrumentation” Slide 18 Gas ionization chambers II Usually filled with Xenon (high Z) under pressure (up to 30 atm) to optimize efficiency Cheap Excellent stability Large dynamic range High spatial resolution Low efficiency BMI I FS05 – Class 3 “CT Instrumentation” Slide 19 Scintillation detectors Scintillating material (phosphor) converts x-ray energy into flashes of visible light Light is measured using photomultiplier tube (PMT) or photo diode (PD) Scintillation materials: For PMT: NaI(Tl), BGO For PD: CdWO4, CsI, rare earth oxides Scintillation material thick enough to provide quantum efficiency ~ 100% Scintillator PD / PMT Electric signal Collimator BMI I FS05 – Class 3 “CT Instrumentation” Slide 20 Photomultiplier tubes (PMT) External photoelectric effect converts light intensity into current of free electrons Electrostatic acceleration of secondary electrons Cascade of secondary electron emission and multiplication on dynodes Signal amplification G = N typ. ~106 (N: no. of dynodes, : gain per dynode ~4) BMI I FS05 – Class 3 “CT Instrumentation” Slide 21 Photodiode Photons create electrons-hole pairs in semiconductor (photoelectric effect) Direct conversion of visible photons into electric energy Generation of photocurrent (~0.5 A / 1 Wopt) requires precision amplifier Packaging in x-Ray CT Detector BMI I FS05 – Class 3 “CT Instrumentation” Slide 22 Advanced Applications BMI I FS05 – Class 3 “CT Instrumentation” Slide 23 5th Generation scanners Exploring the temporal dimension Especially important in cardiovascular (CV) imaging because of fast moving structures Fast slice acquisition Triggering on cardiac cycle High repetition rate BMI I FS05 – Class 3 “CT Instrumentation” Slide 24 Imatron No moving parts Electromagnetically swept electron beam 50 ms (single slice) or 100 ms (multi-slice) scan time imaging of beating heart Developed 1979 at UCSF (Boyd et al.), licensed to Imatron, Inc. BMI I FS05 – Class 3 “CT Instrumentation” Slide 25 Imatron front view BMI I FS05 – Class 3 “CT Instrumentation” Slide 26 Imatron as marketed by GE BMI I FS05 – Class 3 “CT Instrumentation” Slide 27 Single slice sequence (100 ms) Continuous volume scanning (CVS) Step volume scanning (SVS) BMI I FS05 – Class 3 “CT Instrumentation” Slide 28 Multi slice sequence (50 ms) 8 cm axial coverage BMI I FS05 – Class 3 “CT Instrumentation” Slide 29 Triggered acquisition RCA moves at velocities of ~25 – 100 mm/s BMI I FS05 – Class 3 “CT Instrumentation” Slide 30 Imaging examples I Aortic stent Colon w/ 7-mm polyp BMI I FS05 – Class 3 “CT Instrumentation” Slide 31 Imaging examples II Cardiac wall motion "Sharp, motion-free 50 ms images of the heart throughout one entire heart cycle aid physicians in determining and specifying wall motion anomalies." BMI I FS05 – Class 3 “CT Instrumentation” Slide 32 Axial Scans Obtaining Volumetric (3D) Information BMI I FS05 – Class 3 “CT Instrumentation” Slide 33 Volumetric imaging BMI I FS05 – Class 3 “CT Instrumentation” Slide 34 Slice Sensitivity Profile SSP Defined by variation of relative sensitivity along z in the slice center Ideally rectangular (stop-and-shoot profile) -1.5 -1 -0.5 0 0.5 1 1.5 distance to slice center beam with BMI I FS05 – Class 3 “CT Instrumentation” Slide 35 Spiral CT Continuous linear motion of patient table during multiple scans Increased coverage volume / rotation Pitch: Number of slice thicknesses the table moves during one rotation (typically ~1-2) pitch BMI I FS05 – Class 3 “CT Instrumentation” Slide 36 Helical reconstruction Projections for one slice do not lie in one plane Interpolation from data outside the slice plane necessary 1st 2nd 3rd 4th Rotation 0 1st 2nd 3rd 4th Rotation 0 direct data 180 180 complementary data 360 Interpolation: -1 0 1 360 Degree Linear 360 -0.5 0.5 Standard (180 Degree Linear) BMI I FS05 – Class 3 “CT Instrumentation” Slide 37 Complementary data Data sets for view angles 180º apart are identical: Detector array = = Detector array 180º 360º BMI I FS05 – Class 3 “CT Instrumentation” Slide 38 Spiral CT SSP Because of interpolation, SSP deviates from square profile Depending on pitch Full width at half maximum (FWHM) ~ nominal slice width BMI I FS05 – Class 3 “CT Instrumentation” Slide 39 Multi slice spiral scanning I Interweaving multiple helices increased data density Allows higher pitch (faster scan speed) pitch = 4 x single slice pitch BMI I FS05 – Class 3 “CT Instrumentation” Slide 40 Variable Slice Thickness Detector elements (~ 1000 scintillator/PD) are multiplexed to vary slice number and thickness Scan time ~ 0.5 s per rotation BMI I FS05 – Class 3 “CT Instrumentation” Slide 41 Hounsfield units Assign calibrated values to gray scale of CT images Based on measurements with the original EMI scanner invented by Hounsfield Relates the linear attenuation coefficient of a local region to the linear attenuation coefficient of water, W (Eeff = 70 keV) HU original 500 W W HU 2HU original BMI I FS05 – Class 3 “CT Instrumentation” Slide 42